1. Field of the Invention
The present invention relates to a video signal processing circuit, a display apparatus, a liquid crystal display, a projection type display apparatus, and a video signal processing method suitable for improving image quality defects caused by a lateral electric field that occurs in a matrix drive type display panel, for example, a liquid crystal display apparatus or the like.
2. Description of the Related Art
A so-called lateral electric field occurs at a signal boundary region (namely between electrodes of two adjacent pixels) where a potential difference occurs in a video signal supplied to individual pixels in a matrix drive type display apparatus. This lateral electric field disturbs electric fields applied to electrodes of individual pixels, resulting in occurrence of image quality defects. The image quality defects cause shading because of a voltage difference between a drive voltage supplied to a pixel under consideration and that supplied to each of adjacent pixels corresponding to a video signal. FIG. 1A, FIG. 1B, and FIG. 1C show examples in which image quality defects occur.
FIG. 1A shows an example of a display image 1 corresponding to an input video signal and an example of a display image 1A where an image quality defect occurs both on a display apparatus having, for example, 7 (vertical)×7 (horizontal) pixels. 3×5 pixels at a center portion of the display image 1 corresponding to the input video signal have a black level as their luminance and pixels adjacent thereto have a gray level as their luminance. In contrast, pixels 2a to 2c and pixels 2d to 2h that are formed adjacent to the left and below, respectively, of the 3×5 pixels at the center portion of the display image 1A where the image defect occurs have a white-blurring display pattern.
FIG. 1B shows an example of a display image 11 corresponding to an input video signal and an example of a display image 11A where an image defect occurs in a display apparatus having, for example, 7 (vertical)×7 (horizontal) pixels. Likewise, 3×5 pixels at a center portion of the display image 11 corresponding to the input video signal have a black level as their luminance and pixels adjacent thereto have a white level as their luminance. In contrast, pixels 12a to 12e and pixels 12f to 12h that are formed adjacent to the above and the right, respectively, of the 3×5 pixels at the center portion of the display image 11A where an image quality defect occurs have a black-blurring display pattern.
FIG. 1C shows an example of a display image 21 corresponding to an input video signal and an example of a display image 21A where an image quality defect occurs on a display apparatus having, for example, 7 (vertical)×7 (horizontal) pixels. 3×5 pixels at a center portion of the display image 21 corresponding to the input video signal have a gray level as their luminance and pixels adjacent thereto have a white level as their luminance. In contrast, pixels 22a to 22g that are formed adjacent to the above and the right, respectively, of the 3×5 pixels at the center portion of the display image 21A have a black-mixed display pattern.
FIG. 2A and FIG. 2B are schematic diagrams showing a theory of occurrence of an image quality defect phenomenon in a liquid crystal display apparatus. FIG. 2A shows microscopic photos of adjacent pixels 31 and 32. FIG. 2B shows alignments of liquid crystal molecules of the pixels 31 and 32. A lateral electric field 33 occurs between the pixels 31 and 32. The lateral electric field 33 causes the alignments of liquid crystal molecules 34a and 35a that leftward tilt to be disturbed as those of liquid crystal molecules 34b and 35b, respectively. In addition, the lateral electric field 33 causes liquid crystal molecules 34c and 35c that are present in the vicinity of the boundary of the pixel 31 and pixel 32 to be aligned perpendicularly to the lateral electric field 33. Since molecules aligned in parallel or perpendicular to the axis of a polarizing plate occur like the liquid crystal molecule 34c and liquid crystal molecule 35c in the pixels 31 and 32, their transmittances change, resulting in occurrence of black lines 36 and 37. According to such a theory, in the liquid crystal display apparatus, the lateral electric field causes the alignment directions of liquid crystal molecules to rotate and the disturbance of the alignment directions causes a domain-caused image quality defect. When one pixel is composed of three sub-pixels of three primary colors R (Red), G (Green), and B (Blue), a lateral electric field occurs between two sub-pixels of the these primary colors.
Next, with reference to FIG. 3A and FIG. 3B, an outlined structure of a liquid crystal display apparatus will be described. FIG. 3A is an exploded perspective view of a liquid crystal display apparatus. FIG. 3B is an enlarged view of a principal portion of FIG. 3A. As shown in FIG. 3A and FIG. 3B, a liquid crystal display apparatus 40 includes a liquid crystal layer 41, an upper glass substrate 42, a lower glass substrate 44, and polarizing plates 46 and 47. The upper glass substrate 42 and the lower glass substrate 44 are aligned with the liquid crystal layer 41. The polarizing plates 46 and 47 are aligned with the upper glass substrate 42 and the lower glass substrate 44, respectively.
As shown in FIG. 3A and FIG. 3B, a transparent electroconductive film 43 is formed on the upper glass substrate 42. A common electrode that is common in the entire pixel pattern is formed on the upper glass substrate 42. In addition, as shown in FIG. 3A and FIG. 3B, formed on the lower glass substrate 44 are pixel electrodes (pixel patterns) 48n and 48n+1 and thin film transistors (TFTs) 49n and 49n+1 that are switch devices that drive the pixel electrodes (pixel patterns) corresponding to pixels. Moreover, formed on the lower glass substrate 44 are patterns of X electrodes (scanning lines) Xn and Xn+1 that are gate inputs of the thin film transistors 49n, 49n+1 and Y electrodes (signal wires) Yn and Yn+1 that are source inputs thereof. The polarizing plates 46 and 47 are disposed such that axes 46a and 47b of the polarizing plates 46 and 47 are perpendicular thereto.
In such a structure, only liquid crystal molecules 41a and 41b in an area sandwiched by a pixel electrode and a common electrode in the liquid crystal layer 41 are affected by an electric field between the pixel electrode and the common electrode and thereby the their alignments are changed, resulting in functioning as a liquid crystal shutter of one pixel. A lateral electric field occurs between Y electrodes or pixels electrodes of two adjacent pixels due to a potential difference of a video signal supplied to the two adjacent pixels.
Liquid crystal display apparatus are mainly categorized as a perfect vertical alignment type and a tilt alignment type. The perfect vertical alignment type is referred to as so-called VA (Vertical Alignment). In this type, liquid crystal molecules in the liquid crystal layer are aligned perpendicularly to the substrate with an alignment film (not shown) in the state that no voltage is applied to an electrode corresponding to a pixel. In other words, tilt angles θ of the liquid crystal molecules 41a and 41b to the substrate are 90 degrees. If a voltage is applied to an electrode corresponding to the pixel, since the direction in which liquid crystal molecules tilt (alignment direction) is free, the alignment directions of the liquid crystal molecules are not matched.
On the other hand, in the tilt alignment type, an alignment film (not shown) causes liquid crystal molecules of the liquid crystal layer to be aligned such that they tilt in the normal direction of a substrate in the state that no voltage is applied to an electrode corresponding to a pixel and the liquid crystal molecules to be aligned such that they are aligned nearly level with the substrate in the state that a voltage is applied. In other words, as shown in FIG. 3B, pre-tilt angles θ of the liquid crystal molecules 41a and 41b against the substrate are smaller than 90 degrees. When the pre-tilt angles are present in the liquid crystal molecules 41a and 41b, if the liquid crystal display apparatus 40 is viewed from the front (in the direction normal to the substrate), the liquid crystal molecules 41a and 41b tilt in a predetermined direction. When a voltage is applied to an electrode corresponding to a pixel in this state, the directions in which the liquid crystal molecules 34a and 35b shown in FIG. 2B tilt depend on the pre-tilt angles. Since the alignment directions of liquid crystal molecules are decided in one direction, light that transmits through the pixels becomes uniform and thereby the liquid crystal display apparatus displays an image in high quality.
In a liquid crystal display apparatus having such a pre-tilt angle, the direction in which the image quality defect phenomenon occurs also depends on the evaporation direction of liquid crystal molecules. FIG. 4A, FIG. 4B, FIG. 4C show examples of display images corresponding to input video signals in a VA, right-evaporated liquid crystal display apparatus and those where image quality defects occur therein.
FIG. 4A shows an example of a display image 51 of one line (seven pixels) corresponding to an input video signal and an example of a display image 51A where an image quality defect occurs. Three pixels at a center portion of the display image 51 corresponding to the input video signal have a black level as their luminance and pixels adjacent thereto have a gray level as their luminance. In contrast, a pixel 51a that is formed adjacent to the left of the three pixels at the center portion in the display image 51A where an image quality defect occurs has a white-blurring display pattern.
FIG. 4B shows an example of a display image 52 of one line (seven pixels) corresponding to an input video signal and an example of a display image 52A where an image quality defect occurs. Three pixels at a center portion of the display image 52 corresponding to the input video signal have a black level as their luminance and pixels adjacent thereto have a white level as their luminance. In contrast, a pixel 52a that is formed adjacent to the right of the three pixels at the center portion in the display image 52A where the image quality defect occurs has a black-blurring display pattern.
FIG. 4C shows an example of a display image 53 of one line (seven pixels) corresponding to an input video signal and an example of a display image where an image quality defect occurs. Three pixels at a center portion of the display image 53 corresponding to the input video signal have a white level as their luminance and pixels adjacent thereto have a white level as their luminance. In contrast, a pixel 53 formed adjacent to a pixel having a white level on the right of the three pixels at the center portion in the display image 53A where the image quality defect occurs has a black-blurring display pattern.
In contrast, in a left-evaporated liquid crystal display apparatus, the image quality defect phenomenon occurs in a direction opposite to that of the right-evaporated liquid crystal display apparatus shown in FIG. 4A and FIG. 4B. For example, in the display image 51 corresponding to the input video signal shown in FIG. 4A, if the liquid crystal display apparatus is of the left-evaporated type, a pixel 51b that is formed adjacent to the right of the three pixels at the center portion in the image 51A where the image quality defect occurs has a white-blurring display pattern. Thus, although the causes of occurrence of the image quality defects are the same, they differently appear.
In addition, liquid crystal display apparatus have a voltage-transmittance (V-T) characteristic where the transmittance of the liquid crystal layer changes with a voltage applied to a pixel electrode. In color liquid crystal display apparatus, since the VT characteristic differs in each of R (red), G (green), and B (blue), shading of the image quality defective phenomenon differs in RGB.
Although the foregoing liquid crystal display apparatus are of the VA type, twisted nematic (TN) type liquid crystal display apparatus are affected by a lateral electric field. However, since their normally white (NW) and normally black (NB) are different, they differently appear. FIG. 5A and FIG. 5B show display patterns that differ in these types of liquid crystal display apparatus.
FIG. 5A shows an example of a display image 61 composed of 7 (vertical)×7 (horizontal) pixels where an image quality defect occurs in a TN type liquid crystal display apparatus (NW). In a display image corresponding to an original input video signal, 3×5 pixels at a center portion have a black level as their luminance and pixels adjacent thereto have a white level as their luminance. In contrast, in a display image 61 where the image quality defect occurs, pixels 61a to 61g that are formed as five upper pixels and three right pixels of the 3 ×5 pixels at the center portion have a white blurring display pattern.
On the other hand, FIG. 5B shows an example of a display image 62 of 7 (vertical)×7 (horizontal) pixels where an image quality defect occurs in a VA type liquid crystal display apparatus (NB). In the display image 62 where the image quality defect occurs corresponding to the same input video signal as that shown in FIG. 5A, pixels 62a to 62e that are formed adjacent to the above of 3×5 pixels at the center portion and pixels 62f to 62h that are formed adjacent to the right of the 3 ×5 pixels have a black-blurring display pattern.
In the foregoing, the image quality defect phenomenon that occurs, for example, in liquid crystal display apparatus, due to the influence of a horizontal electric field has been described. However, the image quality defect phenomenon due to an influence of a lateral electric field also occurs other than liquid crystal display apparatus. In other words, a similar image quality defect phenomenon occurs in display apparatus where pixels are arranged in a matrix shape on a display panel and voltages are applied to a scanning line and a signal wire of a pixel under consideration such that the pixel under consideration is lit. For example, in organic electroluminescence (EL) display apparatus, a lateral electric field causes the motions of electrons and positive holes in pixels to disturb, resulting in occurrence of an image quality defect. Moreover, in plasma display apparatus, a lateral electric field affects generation of plasma in pixels, resulting in occurrence of an image quality defect.
However, so far, in matrix drive type display apparatus, image quality defects affected by a lateral electric field that occurs between two pixels due to a potential difference of a video signal supplied to individual pixels has been improved. For example, Japanese Unexamined Patent Application Publication No. 2001-59957, referred to as Patent Document 1, discloses a technique that scans pixels at a period shorter than a frame period in synchronization therewith and applies a signal that has been modulated with a pulse width to signal wires. This technique allows liquid crystal to be driven by frame inversion free of flickering and declination.